1. Field of the Invention
The present invention relates to the arrangement and structure of smoothing capacitors in a power inverter such as an inverter.
2. Description of the Prior Art
FIG. 8 is a circuit block diagram of a power inverter for inverting DC power into 3-phase AC power to drive an AC load such as a 3-phase AC motor. In FIG. 8, reference numeral 1 denotes a DC power source for supplying a DC voltage, 2a relatively large-capacity smoothing capacitors for smoothing the voltage of the DC power source 1, 3 a DC input wire connected to the positive (P) sides of the DC power source 1 and the smoothing capacitors 2a, 4 a DC input wire connected to the negative (N) sides of the DC power source 1 and the smoothing capacitors 2a, 5a to 5c switching elements whose collectors are connected to the positive (P) DC input wire 3, and 5d to 5f switching elements whose collectors are connected to the respective emitters of the switching elements 5a to 5c and whose emitters are connected to the negative (N) DC input wire 4. Denoted by 6 is a drive circuit unit connected to the gates of the switching elements 5a to 5f to drive the switching elements 5a to 5f and 7 a control circuit unit for carrying out switching control by outputting a control signal to the drive circuit unit 6. DC power is inverted into 3-phase AC power by the switching operation of the switching elements 5a to 5f under the control of the control circuit unit 7. The switching elements 5a to 5f are transistors, IGBT's or MOSFET's.
Reference numeral 8 represents free wheel diodes whose cathodes are connected to the collectors of the switching elements 5a to 5f and whose anodes are connected to the emitters of the switching elements 5a to 5f to convert 3-phase AC power into DC power. Numeral 9 signifies snubber capacitors which are connected to the collectors of the switching elements 5a to 5f at one end and to the emitters of the switching elements 5a to 5f at the other end to suppress a surge generated in the switching elements 5a to 5d at the time of switching, 10u, 10v and 10w AC output wires connected between the switching elements 5a and 5d, between the switching elements 5b and 5e and between the switching elements 5c and 5f and to which inverted 3-phase (U-phase, V-phase, W-phase) AC power is output, and 11 an AC load, for example, a 3-phase AC motor connected to the AC output wires. The above DC input wires 3 and 4, the switching elements 5a to 5f, the drive circuit unit 6, the free wheel diodes 8, the snubber capacitors 9, and the AC output wires 10u, 10v and 10w constitute a switching power module 12.
The operation of the power inverter will be described hereinunder, taking an electric car as an example. When the car is started or accelerated, the power output of the DC power source 1 which is a battery is changed from DC power to 3-phase AC power to drive the AC load 11 which is a 3-phase AC motor. For the regenerative braking of the car, regenerative power from the AC load 11 is changed from 3-phase AC power to DC power to return it to the DC power source 1.
Since the main characteristics required for the snubber capacitors 9 are excellent frequency characteristics, a film capacitor is generally used as the snubber capacitor 9. The smoothing capacitors 2a suppress fluctuations in the voltage of the DC power source 1 and smoothens a voltage jump or the like. Since sufficiently large capacitance is required for this purpose, an aluminum electrolytic capacitor which can obtain large capacitance with ease is generally used. The control circuit unit 7 outputs a control signal to the drive circuit unit 6 of the switching power module 12 to control the switching elements 5a to 5f. The drive circuit unit 6 and the control circuit unit 7 are circuits for driving and controlling the AC load 11 such as an ordinary 3-phase AC motor or the like and hence, their illustrations are omitted.
FIG. 9 is a side view showing the internal configuration of a general power inverter and FIG. 10 is a plan view showing the internal configuration of the switching power module 12. In FIG. 9, reference numeral 13 denotes a plate-like cooling member, and 14 a case for covering the cooling member 13. The smoothing capacitors 2a, the switching power module 12, a snubber capacitor substrate 15 for mounting the snubber capacitors 9 and a control circuit board 16 for mounting the control circuit unit 7 are stored in the case 14.
The number of the smoothing capacitors 2a is 3 in this case. These smoothing capacitors 2a are aluminum electrolytic capacitors arranged in a row on the cooling member 13, and one end of a wiring board 17 is fixed and electrically connected to the top portion of each of the smoothing capacitors 2a by screws 17a. The other end of the wiring board 17 is fixed and electrically connected to the positive (P) and negative (N) DC input wires 3 and 4 of the switching power module 12 by a screw 18. This wiring board 17 is a copper bus bar, copper plate or the like used to connect the smoothing capacitors 2a to the switching power module 12.
Generally speaking, the snubber capacitor substrate 15 is arranged in the vicinity of the positive (P) and negative (N) DC input wires 3 and 4 and the U-phase, V-phase and W-phase AC output wires 10u, 10v and 10w on the switching power module 12 and fixed and electrically connected to the switching power module 12 by the screws 18.
The package of the switching power module 12 consists of a resin case 21 molded by inserting the positive (P) DC input wire 3, the negative (N) DC input wire 4, the U-phase, V-phase and W-phase AC output wires 10u, 10v and 10w and a drive circuit board connection wire 20, a cover 26 for covering the resin case 21 from above and a switching power module base plate 22.
The positive (P) and negative (N) DC input wires 3 and 4 are arranged in parallel as shown in FIG. 10 and connected to the U, V and W phases of AC output in most cases. Thereby, opposite-direction currents run through the positive (P) DC input wire 3 and the negative (N) DC input wire 4, and the inductance's of the wires offset each other by the mutual induction function of these currents, thereby reducing inductance between the positive (P) DC input wire 3 and the negative (N) DC input wire 4.
An insulated substrate 23 such as a ceramic substrate for mounting the switching elements 5a to 5f and the free wheel diodes 8 and a drive circuit board 24 for mounting the drive circuit unit 6 are stored in the package of the switching power module 12. The switching elements 5a to 5f and the free wheel diodes 8 are fixed on the switching power module base plate 22 through the insulated substrate 23 having a conductor pattern formed thereon by a bonding material such as solder and connected to the positive (P) DC input wire 3, the negative (N) DC input wire 4, the U-phase, V-phase and W-phase AC output wires 10u, 10v and 10w, and the drive circuit board connection wire 20 by a connection conductor 19 such as wire bonding or the like.
The drive circuit board 24 and the drive circuit board connection wire 20 are electrically connected to each other by solder or the like. A gel-like filler 25 is filled in a space between the insulated substrate 23 and the drive circuit board 24 and a resin such as an epoxy resin may be filled over the gel-like filler 25. The gel-like filler 25 protects the switching elements 5a to 5f, the free wheel diodes 8 and the connection conductor 19 to prevent the failure or malfunctioning of the switching elements 5a to 5f caused by moisture or dust. The under surface 24a on the insulated substrate side of the drive circuit board 24 is generally grounded to obtain an electromagnetic shielding effect so as to prevent the drive circuit unit 6 from malfunctioning by switching noise generated by the switching elements 5a to 5f at the time of power inversion. The cooling member 13 for cooling the switching elements 5a to 5f with air, water or oil is installed at the bottom of the case 14 to cool the switching elements 5a to 5f by radiating Joule heat generated from the switching elements 5a to 5f to the cooling member 13 through the insulated substrate 23 and the switching power module base plate 22. Detailed descriptions and illustrations of the installation position and fixing method of the control circuit board 16 are omitted.